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Optional Relationships

A relationship may also be optional. Either end of the relationship can include zero occurrences as an option. This is defined by the business rules of the system being implemented.

Taking the three examples above, the business rules may allow for the following.

  • Not all employees are allocated a company car.
  • A car is defined as a pool car and not allocated to a specific employee.
  • A new department is created but, as yet, there are no employees working within it.
  • A new project is defined but as yet the team has not been established.
  • A new employee starts within the company but, as yet, is not assigned to a project.

Taking the first business rule, graphically this can be shown as:

Entity Model

The circle (O) represents optionality. This will be discussed further later in this section.

Next: Recursive Relationships